We monitored the length of an optical fiber cable stretched between two seafloor anchors separated by 200 m at a depth of 1900 m, 90 km west of Newport, OR, near the toe of the accretionary prism of the Cascadia subduction zone. We continuously recorded length changes using an equal arm Michelson interferometer formed by the sensing cable fiber and a mandrel‐wound reference fiber. A second, nearly identical fiber interferometer (sharing the same cable and housing), differing only in its fiber's temperature coefficient, was recorded simultaneously, allowing the separation of optical path length change due to temperature from that due to strain. Data were collected for 100 days following deployment on 18 October 2015, and showed an overall strain (length change) of −10.7 με (shorter by 2.14 mm). At seismic periods, the sensitivity was a few nε; at tidal periods the noise level was a few tens of nε. The RMS variation after removal of a −79 nε/day drift over the final 30 days was 36 nε. No strain transients were observed. An unexpected response to the varying hydrostatic load from ocean tides was observed with a coefficient of −101 nε per meter of ocean tide height.